The suburban shift toward the Democrats helps explain why they're about to take control of the House. The graphic below shows the shift — and it shows that even the deep-red rural districts elected Republicans with smaller margins than in 2016.

Why it matters: Races tend to be much closer in the suburbs, and that's where Democrats picked up the most seats. With few exceptions, Democrats in conventionally blue districts won by larger margins than Clinton in 2016 and Republicans won by slimmer margins than Trump in 2016.

How to read this chart: A CityLab data project categorized every U.S. House district by the density of its neighborhoods. By comparing the margin victory for Hillary Clinton or Donald Trump in 2016 to the vote margin in the 2018 midterms, we can see which districts shifted to the left or right by comparison.

• Districts with higher Democratic or lower Republican margins point left; those with smaller Democratic or higher Republican margins point right. The color of each arrow shows who won the district in 2018.